Digital data communication via telephone lines as analog circuits have become more prevalent, and modems for performing analog-to-digital and digital-to-analog conversion of signals between terminals and telephone lines have been used widely. To make matching with various signal formats and transmission formats, modems are required to have various operation modes. For example, an operation mode is determined dependent on setting conditions such as a standard specified in Recommendation from Consultative Committee of International Telegraph and Telephone (CCITT) V.21, V.22, V22bis, V.23,V.26bis, V.27ter or V.29, a communication speed given by 300, 1200, 2400, 4800, 7200 or 9600 bps (bits/second), a synchronizing mode given by synchronous or asynchronous mode, the presence or absence of echo protection, etc.
For carrying out data communication via telephone lines, it is essential to match operation modes of modems on both the calling and answering sides with each other. A conventional mode matching method has been practiced by confirming an operation mode of a modem at the other party in advance, and then (a) manually setting an operation mode of one's own modem or (b) replacing one's own modem with another one as required, in conformity with the operation mode of the opposite modem. However, such a conventional method is disadvantageous in requiring the intricate process to set an operation mode after exchanging information between the calling and answering sides prior to connection of a line, and to replace the existing modem with a new one when the setting cannot be completed. It is also difficult to be adaptable for many kinds of operation modes.